The present invention relates to semiconductor lasers and methods of making and using such devices, and more particularly, to distributed feedback (DFB) lasers that operate satisfactorily at low temperatures. The invention also relates to a method of making DFB lasers from semiconductor wafers and the like.
Distributed feedback (DFB) lasers may be used, for example, as sources of signal radiation for optical fiber communications systems, as optical pumps, and as devices for generating coherent optical pulses. In a conventional DFB device, feedback is provided in the longitudinal direction (the emission direction) by an index grating (a periodic array of materials having different optical indices). In another type of DFB laser, a loss grating is used to provide a periodic variation in loss in the longitudinal direction.
There is a need in the art for semiconductor DFB lasers that provide improved performance under conditions, such as low temperature, where they would be positively detuned and suffer degradation in side mode suppression ratio (SMSR, discussed in more detail below).
FIG. 1 shows the relationship between output light intensity I and wavelength xcex for a typical DFB laser, where: xcexm corresponds to the side mode on the short wavelength side of the stopband; xcexo corresponds to the selected lasing mode; and xcexp corresponds to the side mode on the long wavelength side of the stopband. With reference to FIG. 1, the lasing symmetry Lsym for a particular device may be defined as follows:
(1) Lsym=(xcexpxe2x88x92xcexo)/(xcexpxe2x88x92xcexm).xe2x80x83xe2x80x83(1)
According to equation (1), if the lasing symmetry Lsym is smaller than 0.5, then xcexo is closer to xcexp, and if Lsym is greater than 0.5, then xcexo is closer to xcexm. 
The loss function for a DFB laser may be expressed in terms of the coupling coefficient xcexa, as follows:
(2) xcexa=xcexaxe2x80x2+jn, wherexe2x80x83xe2x80x83(2)
xcexaxe2x80x2 represents the real part of the coupling coefficient xcexa, and jn represents the imaginary part of the coefficient xcexa. In a pure index grating DFB laser, jn =0, and there is an equal probability of lasing on the long or short wavelength sides of the stopband, as discussed in more detail below.
The term xe2x80x9cside mode suppression ratioxe2x80x9d(or sub-mode suppression ratio) refers to the ratio of main longitudinal mode power to side longitudinal mode power. Some high capacity fiber optic communications systems require a light source that generates a single longitudinal laser mode. A communications system may require a side mode suppression ratio exceeding 30 dB, for example. For a DFB laser to operate in a single longitudinal mode, the side longitudinal modes should be suppressed to relatively insignificant power levels.
The side mode suppression ratio (SMSR) changes when the laser is tuned to different operating wavelengths. A DFB laser may exhibit acceptable side mode suppression at certain wavelengths, but unacceptable side mode suppression when tuned to other wavelengths. A small tuning change may cause a 10 dB to 20 dB decrease in the SMSR. This is because the relative net threshold gain required for each mode varies as the laser is tuned. When the main longitudinal mode is centered within the reflection characteristics of the laser, side mode suppression is usually optimized. As the device is tuned away from the optimum position, one of the side longitudinal modes is moved closer to the center of the reflection characteristics.
A distributed feedback (DFB) laser having a lasing symmetry Lsym greater than 0.5 provides improved performance (high SMSR) under conditions where it would be positively detuned. An example of a condition where a DFB laser would be positively detuned is at low temperatures (for example, xe2x88x9215xc2x0 C. to xe2x88x9240xc2x0 C.). Thus, the production yield of DFB devices that perform well at low temperature can be increased by forming them in such a way as to increase the likelihood that the devices have lasing symmetries greater than 0.5.
A DFB laser is xe2x80x9cpositively detunedxe2x80x9d when its lasing wavelength (selected mode) is on the long wavelength side of the peak of the material gain spectrum. If the lasing mode is on the long wavelength side, the inherent gain margin is reduced by the relatively large negative gain tilt (dg/dxcex) on the long wavelength side. By adding a small amount of loss to the laser grating (that is, by making jn greater than 0), the likelihood that the device will lase on the long-wavelength side of the stop band is reduced. In other words, a DFB laser with a loss grating is more likely to have a lasing symmetry Lsym greater than 0.5. If more devices from a wafer are produced with Lsym greater than 0.5, then the yield to the low temperature performance requirement is increased.
The present invention relates to a method of making low threshold semiconductor DFB lasers for low temperature operation. The method includes the steps of: (1) providing a multi-layer semiconductor structure, such as a wafer, having an active layer, a loss grating, and a spacer layer; and (2) cleaving the multi-layer structure such that opposed facets intersect the loss grating. The cleaving process is such that the precise location at which a particular facet intersects the loss grating cannot be controlled in advance. As discussed in more detail below, the performance characteristics of the laser depend in part on the phase relationships between the randomly determined locations of the facets and the periodic structure of the loss grating.
According to a preferred embodiment of the invention, the facets may be covered by highly reflective and anti-reflective coatings, and other structures such as claddings, substrates, electrodes, etc. may also be provided.
According to another aspect of the invention, the loss grating may be grown on a semiconductor substrate by a metal organic chemical vapor deposition (MOCVD) process. In a preferred embodiment of the invention, the loss grating has a high As content and therefore is well suited for reliable growth according to an MOCVD process. In another embodiment of the invention, the loss grating may be formed by molecular beam epitaxy.
According to another aspect of the invention, lasers are constructed to provide high side mode suppression under conditions where they would be positively detuned. An example of such conditions is where the operating temperature is less than about minus fifteen degrees Celsius (xe2x88x9215xc2x0 C.).
The present invention also relates to a DFB laser for low temperature single mode operation, including: an active layer for producing optical gain; a loss grating for shifting the emission spectrum to the short wavelength side of the stopband; and a spacer layer located between the active layer and the loss grating. According to a preferred embodiment of the invention, the device has a lasing symmetry Lsym greater than 0.5. Consequently, the device does not lase on the long wavelength side of the stopband under low temperature conditions. The low operating temperature may be, for example in the range of from xe2x88x9215xc2x0 to xe2x88x9240xc2x0 C.
According to another aspect of the invention, a DFB laser is constructed to have a high SMSR, to operate at a low threshold, and to exhibit minimal mode hopping, all at low temperatures.
According to another aspect of the invention, a device is provided that operates in a single longitudinal mode, with a large gain threshold difference, and that has facet-reflectivity-independent parameters, and low sensitivity to feedback.
In a preferred embodiment of the invention, the problems of the prior art are overcome by using a loss grating to shift the distribution of lasing modes, generated by random facet phases, to the short wavelength side of the stopband. The loss grating counteracts the effect of the negative gain tilt that occurs in DFB lasers due to positive detuning at subzero temperatures. According to the present invention, temperature-induced variations of the operational characteristics of a DFB laser are minimized.